THE ABSORPTION OF MINERAL ELEMENTS 67 



dispersed in the crust of the earth in such small amounts that they 

 may not be discovered by ordinary analysis, but they may be pres- 

 ent in a considerable concentration in plants. Bromine and iodine, 

 for example, accumulate in large quantities in the seaweeds. 

 Some geologists, as for instance Vernadsky, attribute to the 

 accumulating power of the organisms a significant role in the 

 general circulation of the rare elements on the earth's surface. 



The following elements are most often found in the ash of 

 plants: K, Na, Ca, Mg, Fe, Mn, Zn, B, Al, Si, P, S, and CI. It 

 does not mean, however, that all of these are really necessary. 

 It has been seen that the normal development of plants may be 

 obtained in water cultures on four- or even three-salt solutions 

 which contain only K, Ca, Mg, Fe, S, P, and N. 



To these seven principal elements, whose necessity for the nutri- 

 tion of the plant was proved in the middle of the last century by the 

 classical experiments of Knop and Sachs, several others have been 

 added: manganese, zinc, boron, and silica. These four elements 

 were formerly regarded as superfluous, accumulating in the plant 

 only by virtue of the fact that they are dissolved in the water that 

 is absorbed by plants from the soil. This increase in the number of 

 necessary elements was the result of water cultures conducted with 

 salts carefully purified from the smallest amounts of impurities 

 by the exact methods of modern chemistry, and used in contain- 

 ers coated on the inside with a paraffin layer, in order to prevent 

 the solution of elements found in the glass. All of these sub- 

 stances, similar to iron, are required by plants only in exceedingly 

 small quantities. With the usual method of water cultures there 

 is no need to introduce them into the nutrient solution. Traces 

 are always found in the common "chemically pure" reagents or 

 are leached from the glass walls of the container. It is not improb- 

 able that further investigations with more carefully purified rea- 

 gents will reveal the necessity of still other elements for the 

 normal development of the plant, elements which now are regarded 

 as accidental and useless admixtures. 



It must be noted that the relative amount of one or another 

 element in the ash of the plant by no means indicates the degree 

 of its necessity. Thus, the plant can do without sodium, an ele- 

 ment constituting a perceptible part of the ash of plants and even 

 accumulating abundantly in some of them. On the other hand, 

 the presence of traces of manganese and boron is absolutely neces- 



